Rainwater Harvesting: Cycles of Concentration & Scaling

There are several process advantages to using rainwater as the feed water source for cooling towers. The first advantage is that because the total dissolved solids are so low in rainwater, the result is that with rain water you can run higher cycles of concentration when compared to City utility water. Higher cycles of concentration mean that the process uses less make-up water. Less make-up water means lower water bills. Using less make-up water also mean that if you are using a chemical in the cooling tower you will use significantly less chemical as it will stay in the cooling system longer.

Let’s look at cycles of concentration and how it affects cooling tower operation. As pure water is evaporated, minerals and solids are left behind. The term cycle of concentration compares the level of solids in the recirculating water to the level of solids in the make-up water. For example, if the solids in the make-up water are 125 u-Siemens/cm and the solids in the recirculating water are at 500 u-Siemens/cm then the cycles of concentration are four.

Cycles =

Bleed Off Concentration u-S/cm

———————————————–

Make-Up Water Concentration u-S/cm

Cycles of concentration then is a measure of the degree to which dissolved solids are being concentrated in the circulating water. When the dissolved solids reach a certain value (generally 1800 u-Siemens/cm) then a solenoid valve will open which will allow water high in dissolved solids to exit the system (bleed off). At the same time, another solenoid valve is opening which allows water low in dissolved solids (make-up water) to enter the system.

This process of bleeding off water high in dissolved solids and replacing it with water low in dissolved solids continues automatically during the uninterrupted operation of the cooling tower. If solids were not removed from the system, all the water would evaporate, and the cooling tower would turn into one large rock from dissolved solids.

The other very significant advantage to using rainwater for cooling tower applications is that since the rainwater is so low in dissolved solids (5u-Siemens/cm) relative to City utility waters, there are no constituents in the water to cause scale. Gone are the inorganics, including calcium, magnesium, silica, sulphur, iron and other metals. With no cations and anions to cause scale on heat transfer systems (cooling tower condenser, heat exchangers, etc), the cooling tower will operate scale free which means no wasted energy due to scaled heat transfer surfaces. Consider the following two water sources:

Parameter Rainwater Typical City Utility Water

Total Dissolved Solids 5 u-S/cm 81 to 274 u-S/cm

Hardness (calcium/magnesium) Non-Detectable 68.4 to 256.5 mg/l

By using rainwater harvesting systems to capture fresh rain water, we can utilize it in our industrial processes at a significantly lower price than City utility water. In turn, we will protect our dwindling supplies of drinking water and exercise good stewardship over our natural resources.